Calculation of energies for any configuration of atoms enabled the evaluation of interatomic force constants between displaced atoms, and thereby phonon spectra. Initially this was accomplished
for each phonon independently, but capabilities were rapidly extended, especially by applying density functional perturbation theory (allowing higher order to be obtained analytically) and Wannier function techniques.[1]}, [2]}, [3]} The outcome was true harmonic phonon frequencies through formalism making use of infinitesimal atomic displacements. The change in the electronic potential due to an atomic displacement is the root factor in EP coupling, and calculation of EP matrix elements was achieved only around 2000.
